arXiv subjects

Ankit Dhanuka

Publications and source records attributed to Ankit Dhanuka.

6 recordsLinked to original sources

arXiv2025

Ecosystems as adaptive living circuits

Unlike many physical nonequilibrium systems, in biological systems, the coupling to external energy sources is not a fixed parameter but adaptively controlled by the system itself. We do not have theoretical frameworks that allow for such adaptability. As a result, we cannot understand emergent behavior in living systems where structure formation and non-equilibrium drive coevolve. Here, using ecosystems as a model of adaptive systems, we develop a framework of living circuits whose architecture changes adaptively with the energy dissipated in each circuit edge. We find that unlike traditional nonequilibrium systems, living circuits exhibit a phase transition from equilibrium death to a nonequilibrium dissipative state beyond a critical driving potential. This transition emerges through a feedback mechanism that saves the weakest edges by routing dissipation through them, even though the adaptive rule locally rewards the strongest dissipating edges. Despite lacking any global optimization principle, living circuits achieve near-maximal dissipation, with higher drive promoting more complex circuits. Our work establishes ecosystems as paradigmatic examples of living circuits whose structure and dissipation are tuned through local adaptive rules.

q-bio.PE↗

arXiv2025

Excitability and travelling waves in renewable active matter

Activity and renewability are distinctive features of living matter, and constitute a new class of materials that we term renewable active matter. A striking example is the cell cytoskeleton, where myosin filaments bind to the actin meshwork, apply contractile stresses and undergo continual stress/strain dependent turnover, thus acting as both force generators and sensors. As a consequence of nonreciprocity, arising from the independence of action and response, such living matter exhibits unusual mechanical properties like, segregation without attraction, fragility and force chains. Here we show that the interplay between activity and turnover gives rise to mechanical excitability in the form of travelling waves and pulses, and spatiotemporal chaos. We provide a systematic study of the nucleation, movement and shape of the travelling pulse, and present a boundary layer analysis to establish the existence of homoclinic orbits. Our analytical results are supported by detailed numerical analysis of the governing partial differential equations. This study has implications for the observed mechanical excitability in a variety of cellular contexts such as in isolated adherent cells and confluent cells within tissues.

cond-mat.soft↗

arXiv2022

Quantum Gravity signatures in late time Universe

In the cosmological settings, Quantum Gravity effects are typically understood to be limited towards very early phase of the universe, namely in the pre-inflationary era, with limited signatures spilling over into the succeeding inflationary era which gradually fade away as the universe transits into the subsequent radiation and matter driven expansion. In general scenarios as well, the imprints of possible quantum character of gravitons are typically so feeble that they are expected to remain buried under overwhelming noise from other stronger processes. In this work, we demonstrate that quantized gravitational perturbations cause strong observable effects in cosmological settings post the Last Scattering Surface (LSS) more prominently than any other classical or quantum processes. This counter-intuitive effect is facilitated by the fact that the quantum correlators of the gravitons {\it grow divergently large in the matter dominated era} unlike any other background fields leading to an abrupt rise in the processes mediated by correlators of quantum gravitons, which otherwise is way too feeble. The transitions between spherical harmonics states of a hydrogen atom guided by gravitons in a particular phase of the late time era of the universe, provides an example of such a process. Thus the newly formed atoms post LSS are expected to get excited from the vacuum fluctuations of gravitons quite efficiently in this era and also get de-excited to convert the vacuum fluctuations to an additional channel of energy influx into the evolution history. Such unavoidable processes provide an avenue of quantum graviton mediated process becoming important in the late time era, which has many interesting implications.

gr-qc↗

arXiv2022

Unruh deWitt probe of late time revival of quantum correlations in Friedmann spacetimes

Unruh deWitt detectors are important constructs in studying the dynamics of quantum fields in any geometric background. Curvature also plays an important role in setting up the correlations of a quantum field in a given spacetime. For instance, massless fields are known to have large correlations in de Sitter space as well as in certain class of Friedmann-Robertson-Walker (FRW) universes. However, some of the correlations are secular in nature while some are dynamic and spacetime dependent. An Unruh deWitt detector responds to such divergences differently in different spacetimes. In this work, we study the response rate of Unruh deWitt detectors which interact with quantum fields in FRW spacetimes. We consider both conventionally as well as derivatively coupled Unruh deWitt detectors. Particularly, we consider their interaction with massless scalar fields in FRW spacetimes and nearly massless scalar fields in de Sitter spacetime. We discuss how the term which gives rise to the infrared divergence in the massless limit in de Sitter spacetime manifests itself at the level of the response rate of these Unruh deWitt detectors in a wide class of Friedmann spacetimes. To carry out this study, we use an equivalence between massless scalar fields in FRW spacetimes with massive scalar fields in de Sitter spacetime. Further, we show that while the derivative coupling regulates the divergence appearing in de Sitter spacetime, it does not completely remove them in matter dominated universe. This gives rise to large transitions in the detector which can be used as a probe of setting up of large correlations in late time era of the universe as well. We show that the coupling of hydrogen atoms with gravitational waves takes a form that is similar to derivatively coupled UdW detectors and hence has significant observational implications as a probe of late time revival of quantum correlators.

gr-qc↗

arXiv2022

Fluctuations in the stress energy tensor of spinor fields evolving in general FRW spacetimes

In this work, we study quantum fluctuations in the stress energy tensor of spinor fields evolving in general FRW spacetimes. We quantify these fluctuations by the noise kernel of spinor fields. For the particular case of de Sitter spacetime, we place the spinor field in what is called the fermionic Bunch Davies vacuum and study the variation of the noise kernel with the mass of the field. We then make use of the conformal invariance of massless spinor fields and employ an equivalence that relates a massless spinor field in any given FRW spacetime with a corresponding massless spinor field in de Sitter spacetime. Using this equivalence, we study the behaviour of the noise kernel of massless spinor fields in general FRW spacetimes while placing the fields in the Bunch Davies vacuum of the corresponding massless spinor field of the de Sitter spacetime. We extend this analysis to the coordinate invariant quantity of the energy energy correlator. We compare these results for spinor fields in considered spacetimes with the analogous results for scalar fields. This study helps us better understand whether the spin 1/2 fermionic matter remains strongly correlated in considered spacetimes leading to considerable backreaction or not.

gr-qc↗

arXiv2020

Stress energy correlator in de Sitter space-time : its conformal masking or growth in connected Friedmann universes

Semiclassical Physics in gravitational scenario, in its first approximation (1st order) cares only for the expectation value of stress energy tensor and ignores the inherent quantum fluctuations thereof. In the approach of stochastic gravity, on the other hand, these matter fluctuations are supposed to work as the source of geometry fluctuations and have the potential to render the results from 1st order semiclassical physics irrelevant. We study the object of central significance in stochastic gravity, i.e. the noise kernel, for a wide class of Friedmann space-times. Through an equivalence of quantum fields on de Sitter space-time and those on generic Friedmann universes, we obtain the noise kernel through the correlators of Stress Energy Tensor (SET) for fixed co-moving but large physical distances. We show that in many Friedmann universes including the expanding universes, the initial quantum fluctuations, the universe is born with, may remain invariant and important even at late times. Further, we explore the cosmological space-times where even after long times the quantum fluctuations remain strong and become dominant over large physical distances, which the matter driven universe is an example of. The study is carried out in minimal as well as non-minimal interaction settings. Implications of such quantum fluctuations are discussed.

gr-qc↗